Currently, terrestrial reception stations generally use an array of mobile parabolic antennas to track satellites. An example of such a reception station is represented in FIG. 1.
A problem with antennas of this type is that they are complex to implement. They also suffer from difficulties of speed of change of pointing, which can lead to performance degradations. Moreover, in reception stations, the number of antennas is very limited due to cost. Moreover the increase in gain in the antenna is accompanied by an increase in the dimension of the size of the antenna, and therefore an increase in complexity and cost.
It is known to use an active phase-control antenna to replace one or more mobile antennas. An active phase-control antenna is composed of a plurality of radiating sub-elements each having a phase shifting circuit. In such an antenna, the wave emitted or received by each of the sub-elements interferes with the wave of the others and a beam is produced through the sum of these constructive interferences. Moreover, by varying the phases and the amplitudes between each of the sub-elements, the beam may be steered towards a specific direction.
One of the difficulties related to the use of an active antenna, for example for tracking and communicating with satellites, is the control of the gain of its sub-elements and particularly in the frequency band in which they are used.
If for example, in a particular direction, a phase shift is created or a gain is deformed, the phase law which will be optimal for pointing in this particular direction will not be the same as if there had been no deformation. It should be noted that this deformation may vary over time.
It is known that the disturbing elements which may give rise to phase variations are elements of the order of a wavelength. Therefore, when the frequency band is low, for example a wavelength of the order of some twenty or so centimeters, a multitude of elements may disturb the operation of the antenna. Moreover, the antenna is also prone to the effects of ionospheric scintillation for example.
To use an active antenna in a precise manner it is therefore necessary to perform a calibration of the antenna, that is to say contrive matters so that the phase and amplitude law between the various sub-elements is truly optimal for beam formation so as to guarantee optimum antenna gain. This calibration may be performed definitively for example in the factory but the precision of beam formation will not be optimal. This calibration can also be performed permanently but this operation is fairly expensive.
The use of a single active antenna to replace several mobile antennas therefore generally suffers from the difficulties of calibration which lead either to an appreciable degradation of the gain if it is done poorly, or to the putting in place of complex and expensive calibration systems to obtain optimum gain. The use of active antennas for tracking constellations at wavelengths of greater than 30 cm is therefore limited at present.